THE POISON GARDEN website

Pontifications on Poison

Being some ramblings on events associated with poisonous plants.

Sunday 13th November 2011

I’ve been reading a paper, from 2008, written by a trio of
evolutionary biologists from America, Canada and Germany.
I’ve
written before 29th Oct that I’m not always happy with the
way people describe the process of evolution when referring to
the properties plants currently have.

It seems to me that evolutionary biologists are keen to
explain absolutely every interaction between different species
in terms of evolution but I don’t believe that to be the case.
I’ll just repeat that I don’t for a moment doubt that evolution
exists and works. I just don’t think there is an evolutionary
explanation for everything.

Take
Heracleum mantegazzianum, giant hogweed, as an example. As
far as has been established, the production of furocoumarins by
plants is a response to the presence of a root fungus. The plant
produces the furocoumarins to prevent the root fungus from doing
so much damage that the plant dies. In evolutionary terms, the
two organisms have arrived at a balance where all the versions
of the plants that didn’t produce enough furocoumarins have died
out and all the versions of the fungus that weren’t put off by
the furocoumarins killed their host and, thus, died themselves.

But, furocoumarins act on the skin of humans to make it
sensitive to light resulting in burns. The burning doesn’t
happen for up to 24-48 hours after the contact so it doesn’t
stop a human from completely cutting down a giant hogweed.
There’s no way to claim that the plant benefits from making
human skin so light sensitive as to cause pain and harm.

There’s no evolutionary mechanism in this. The evolution is
going on between the plant and the root fungus. What happens to
humans is just collateral damage, in today’s jargon. But, going
from the paper I referred to above, I would expect evolutionary
biologists to want to prove there was some purpose in this
interaction.

Not that the paper had anything to do with phytophototoxicity.
What it was looking at was the effect of psychoactive substances
on humans. The authors acknowledged that current theories of
what psychoactive substances do create a paradox in terms of
evolutionary interaction between plant and humankind but, rather
than say there is no evolutionary interaction, they proceed to
try and unravel the paradox.

Back in June 19th, I briefly explained what psychoactive
substances do for humans;

‘To survive, human beings have to do three
things; eat, reproduce and protect and provide for our
dependants. In order to encourage us to undertake these
activities we have a reward system that makes us feel good when
we eat a satisfying meal, engage in the sex act or apply effort
to achieve some end like providing a shelter. Like everything
going on in our bodies, this ‘feeling good’ response is the
result of complex chemical reactions. Psychoactive substances
reproduce those chemical reactions so that it is possible to
achieve the feeling of reward without engaging in the activity.’

In June, I was interested in why this mechanism means you can
never stop some people from wanting to use psychoactive
substances but, today, I want to look at the evolutionary
aspects. We’ve evolved to have these reward systems as a way of
ensuring we reproduce. Something that means we don’t need to
make the efforts to advance our species in order to achieve the
reward sensation runs counter to evolution.

Look at it from the plant’s point of view. Plants have
evolved to have substances that help them stay alive to the next
generation. Some of those substances are toxic to herbivores so
that a balance has been reached deterring herbivores from
consuming so much of a plant as to prevent its bearing fruit and
reproducing. Substances that are psychoactive to humans
encourage consumption of the plant and run counter to the whole
idea of plant components being part of the plant’s defence
against getting consumed. Again, the normal ideas about how
evolution works and what it achieves are stood on their head.

But, rather than say, evolution isn’t involved in the use of
psychoactive plant-derived substances, the evolutionary
biologists who authored the paper I was reading offer some
bizarre ways in which evolution can be made to fit the
relationship.

I’ve read elsewhere, the suggestion that plants make
substances that humans like so that we will take more care of
them and increase their population. This has been said about
Cannabis sativa and
on 29th
June I wrote about the flaws in that argument. Cannabis
produces its sticky resin as an aid to pollination not because
it wants to be loved by man.

The case is made that finding the evolutionary drivers that
make these plant-derived substances psychoactive and lead to
addiction in at least some of the people who use them will give
us help in determining the causes of addiction.

But I don’t see that searching for something that isn’t there
can be helpful.